Electrically-operated display devices

ABSTRACT

An electrically-operated display device comprises an evacuated, or alternatively gas-filled, housing and three electric filaments disposed within the housing to provide an illuminated display when energized to incandescence. The filaments are carried within the housing on a planar panel that is supported from a base of the housing on terminal pins so as to have an upper face in close proximity to, and exposed for view through, a transparent wall of the housing. The filaments, connected at either end to the terminal pins, are threaded through holes spaced across the surface of the panel such that successive spaced parts of the filaments extend across the panel surface to define discrete portions of the symbol to be displayed. The display device is mounted in head-up display equipment of an aircraft as the source of an auxiliary stand-by display that may be selected for projection via a collimator onto a partially transparent reflector in the line of sight of the aircraft pilot. This auxiliary display is interchangeable with the main flight display provided by a cathode-ray tube in the event of failure of the tube, pilot operation of a selector handle in one direction retracting the tube from the collimator and then pivoting the stand-by display into the resultant space along an arcuate path substantially normal to the direction of retraction of the tube.

United States Patent 11 1 McAinsh et al.

[ ELECTRICALLY-OPERATED DISPLAY DEVICES [76] Inventors: Kenneth Gordon McAinsh, 24

Fairfield Dr., Camberly; Ralph David Smith, 31 Manor Rd., Walton-on-Thames; Edward Lawrence Lamb, Sandringham Dr., Ashford, all of England 22 Filed: Nov. 23, 1971 21 Appl. No.: 201,476

[30] Foreign Application Priority Data Nov. 24, 1970 Great Britain 55,703/70 [52] US. Cl. 340/27 NA, 313/109.5, 340/336, 340/366 R, 340/366 F, 340/378 B, 353/11, 353/30 [51] Int. Cl. G08b 5/36 [5 8] Field of Search 340/27 R, 27 NA, 340/366 R, 366 D, 366 F, 378 R, 378 B, 336; 313/1095; 353/11, 12, 30; 343/5 EM [56] References Cited UNITED STATES PATENTS 2,878,418 3/1959 Garfinkel et a1 313/1095 X 3,604,971 9/1971 Tracy 313/1095 3,564,325 2/1971 Bonnette et al. 340/336 X 2,470,912 5/1949 Best et al. 340/27 NA -3,317,906 5/1967 Baldridge 340/366 R 3,408,523 10/1968 Demarest et a1 340/336 X 3,486,816 12/1969 Streeter 353/11 3,593,259 7/1971 Stormo 340/27 NA f! 7 l i I I 1 I 4 ?fl\ E "2\ [5/ I 7a 7 4 4 "3 Oct. 9, 1973 Primary Examiner-David L. Trafton Att0rney-Willaim D. Hall et al.

[5 7] ABSTRACT An electrically-operated display device comprises an evacuated, or alternatively gas-filled, housing and three electric filaments disposed within the housing to provide an illuminated display when energized to incandescence. The filaments are carried within the housing on a planar panel that is supported from a base of the housing on terminal pins so as to have an upper face in close proximity to, and exposed for view through, a transparent wall of the housing. The filaments, connected at either end to the terminal pins, are threaded through holes spaced across the surface of the panel such that successive spaced parts of the filaments extend across the panel surface to define discrete portions of the symbol to be displayed. The display device is mounted in head-up display equipment of an aircraft as the source of an auxiliary stand-by display that may be selected for projection via a collimator onto a partially transparent reflector in the line of sight of the aircraft pilot. This auxiliary display is interchangeable with the main flight display provided by a cathode-ray tube in the event of failure of the tube, pilot operation of a selector handle in one direction retracting the tube from the collimator and then pivoting the stand-by display into the resultant space along an arcuate path substantially normal to the direction of retraction of the tube.

15 Claims, 9 Drawing Figures PATENTED BET 5 SHEET 5 UF 6 ELECTRICALLY-OPERATED DISPLAY DEVICES This invention relates to electrically-operated display devices.

According to one aspect of the present invention there is provided an electrically-operated display device including a gas-tight housing which has a transparent wall portion of substantially planar form and which encloses a substantially planar panel that is mounted within the housing in face to face opposition and close proximity to the planar wall portion, and wherein the panel carries an electric filament which at least along a part of its length extends across the panel intermediate the panel and the wall portion in the form of a symbol to be displayed, and which is adapted to be energized from outside the housing to provide an illuminated display of said symbol through the wall portion.

The symbol may be simply a line in either dashed or continuous outline.

The electric filament may be constituted by an electrical resistance wire or the like which is threaded through holes in the panel, or by a printed circuit formed on the surface of the panel adjacent to said wall portion of the housing.

The housing may be evacuated or gas filled.

The gas-tight housing may include a base of substantially planar form which carries a plurality of electric terminals, the terminals extending through the base and being electrically connected to respective ends of the electric filament.

The said panel may extend substantially parallel to the base of the housing, and, in such a case, the panel may be mounted on the electric terminals.

Alternatively, the said panel may extend transversely of the base of the housing, and, in this case, the panel may be directly mounted on the base of the housing.

The display device may include more than one electric filament each of which is'carried by the said panel.

According to a further aspect of the present invention there is provided a head-up display apparatus for a craft, wherein collimator means is arranged to project on to a partially-transparent reflector the display provided by a display-producing means so that the display projected is visible to an observer in the craft viewing a distant scene through the reflector, and wherein the display-producing means comprises an electricallyoperated display device as aforesaid.

Preferably the display on the reflector is provided by a selected one of two display-producing means, the selection of the particular display-producing means involved being effected by selection means that is. operable to move the selected display-producing means to the collimator means and the other display-producing means from the collimator means.

The movement of the two display-producing means may be effected sequentially.

A first of the display-producing means may be mounted for movement longitudinally from the collimator means so as thereby to provide a space between this display-producing means and the collimator means and a second of the display-producing means may be mounted for movement into the said space along a path substantially normal to the direction of said movement of the first display-producing means. The said path may be an arcuate path.

The two display-producing means may be arranged to provide respectively a main display of information and an auxiliary display of information that can be used in the event of failure of the main display, and, in such a case, said first display-producing means may be arranged to provide the main display of information and said second display-producing means may be arranged to provide the auxiliary display of information.

The said second display-producing means may comprise the said electrically-operated display device.

Various forms of electrically-operated display devices for head-up display apparatus in accordance with the present invention will now be described, by way of example, having reference to the accompanying drawings in which:

FIG. I is a side elevation, partly in section, of part of one form of head-up display apparatus;

FIG. 2 is a plan view of FIG. 1 partly in section on the line II-II;

FIG. 3 is an end elevation on the line III-III of FIG.

FIG. 4 is a perspective view of a sleeve shown in FIGS. 1 to 3;

FIG. 5 is a sectional side view of an electricallyoperated display device which provides a stand-by display of information for the head-up display apparatus;

FIG. 6 is a sectional plan view on the line VI-VI of FIG. 5;

FIG. 7 is a fragmentary sectional view on the line VII- VII of FIG. 6;

FIG. 8 is a sectional side view of an alternative form of electrically-operated display device which may be used toprovide the stand-by display of information for the head-up display apparatus; and

FIG. 9 is a sectional view on the line IXIX of FIG. 8.

Referring to FIGS. 1 and 4, the head-up display apparatus includes a unit 10 for producing a main display of information and a unit 11 for producing a stand-by display of information for use in the event of failure of the main display. The units 10 and 11 are housed in a generally tubular elongated housing 12 of aluminum.

The housing 12 is mounted in an aircraft, by means not shown, immediately above the instrument panel and in front of the pilot with the longitudinal axis of the housing 12 extending lengthwise of the aircraft. The housing 12 has secured to one end thereof a collimator 13 for projecting the display of information produced by either of the units 10 and 11 on to an inclined partially-transparent reflector (only part of which is shown) mounted above the collimator 13 on a frame carried by the housing 12. The reflector 100 is disposed in the line-of-sight of the pilot through the windscreen of the aircraft and is inclined so that the display of information projected on to the reflector 100 appears in the line-of-sight of the pilot. Thus, the pilot views the collimated image of the display of information projected on to the reflector 100 against a background of the scene through the windscreen of the aircraft, and does so without any need for refocussing of his eyes.

The unit 10 includes a generally tubular elongated casing 14 in which a cathode-ray tube 15 is mounted with the face of the tube 15 in alignment with the end of the casing 14 adjacent the collimator 13. The casing 14 is slidably mounted within the housing 12 to vary the spacing between the face of the tube 15 and the collimator 13. Rotational movement of the casing 14 relative to the housing 12 is prevented by a pin 16 that is carried by the housing 12 and projects into a groove 17 formed in the casing 14 and extending longitudinally thereof. The housing 12 and the casing 14 are shaped to provide therebetween an annular space 18 which is centered on the longitudinal axis of the housing 12, the annular space 18 being bounded at the end nearest to the collimator 13 by a peripheral shoulder 180 on the casing 14 and bounded at the other end by a peripheral shoulder 18b on the housing 12. The annular space 118 houses a plurality of springs 119 which are spaced apart around the casing 14 and are mounted to urge the face of the tube towards the collimator 13.

The unit 11 includes a pair of generally arcuateshaped, metal plates 20 and 2B which are spaced apart and which are joined at their edges by a side wall 22 that extends completely round the periphery of the plates 20 and M. The unit 111 is pivotally mounted on a tubular pivot-pin 23 which extends through holes formed in one end of the plates 263 and 21, the ends of the pin 23 extending outwardly from the unit 11 and being rotatably mounted in holes formed in flanges 24 and 25 on respectively the housing 12 and a' housing 26 of the collimator 113. The unit 11 is movable about its pivot between the position shown in FIGS. 1 and 3 where the unit 11 is disposed between the collimator 13 and the face of the tube 15, and a position, shown in dashed outline in FIG. 3, which allows the unit lit) to be advanced to bring the face of the tube 15 into abutment with a washer 27 carried by the collimator 13.

The unit 11 also includes an electrically-operated display device 29 which is disposed between the plates 2t) and 21. The device 29 comprises a gag-tight housing in which is disposed a filament which is shaped so that when heated to incandescence it provides a stand-by display of information of the general form shown at 28 in FIG. 3. The ends of the electric filament are connected to terminal pins which project from the housing to enable the device 29 to be plugged into a base member having terminals which electrically engage respective terminal pins of the device 29. The terminals of the base member are connected to respective terminals of an electrical connector 33 to facilitate the supply of electric power to the electric filament of the device 29.

During normal operation of the head-up display apparatus the unit 11 is disposed in the position shown in dashed outline in FIG. 3 and the unit 141 is positioned so that the face of the tube 15 abuts the washer 27. In the event of failure of the display of information on the screen of the tube 15, the unit 10 is moved to the position shown in FIG. 1 and thereafter the unit 111 is moved into the position shown in FIGS. ii and 3 so that the stand-by display of information is projected on to the partially-transparent reflector.

The movement of the units 10 and ill is effected by selection means comprising an arm 34 which is rotatably mounted on a boss 36 carried by the housing 26 so as to enable the arm 34 to be moved between the position shown in full outline in FIG. 1 and the position shown in dashed outline in FIG. 1 and reference 34A. The arm 34 carries a gear segment 37 whose teeth engage a bevel gear-wheel 38 (FIG. 2) carried by a shaft 39 which is rotatably mounted in bearings 4&1 and 411 disposed in the flange 24. The shaft 39 also carries a spur gear-wheel 42 for transferring rotational movement of the bevel gear-wheel 38 to a sleeve 4-3 which has teeth formed in its outer circumferential surface for engaging the teeth on the spur gear-wheel 42 and which is mounted for rotational movement in the housing i2 about the longitudinal axis thereof. Thus, movement of the arm 34 between the positions shown in FIG. 1 causes the sleeve 43 to be angularly displaced about the longitudinal axis of the housing 12. The sleeve 43 is mounted in the housing 12 to prevent relative axial displacement of the sleeve 43 and the housing 12.

The sleeve 43, which is shown in more detail in FIG. 4-, has a pair of diametrically-opposed slots 44 formed in its inner circumferential surface which are engaged by respective pins 45 carried by the casing 14. Each slot 44 has a curved portion 46 which forms part of a helical spiral centered on the longitudinal axis of the housing 1.2 and a portion 47 which lies in a plane normal to the longitudinal axis. The sleeve 43 carries a pin 4% (F16. 4) which extends outwardly from the end of the sleeve 43 adjacent to the collimator l3 and engages a slot 49 formed in the plate 2d of the unit 11. The slot 4-9 is provided at the end of the plate 20 remote from the 23 and has a curved portion 50, and a straight portion 31 disposed at the end of the curved portion 50 remote from the pin 23. The curved portion 50 is in the form of an are which, when the unit 11 is dispoed in the position shown in dashed outline in FIG. 3, is centered on the longitudinal axis of the housing 12, while the straight portion 511 diverges outwardly from the end of the curved portion 56 remote from the pin 23 and subtends an angle of approximately forty degrees thereto.

When the units MI and 11 are in their normal positions, that is with the face of the tube 15 abutting the washer 27 and the unit lli in the position shown in dashed outline in FIG. 3, the arm 34 is in the position referenced 34A, the pin 48 is disposed at the end of the curved portion 911 of the slot 49 remote from the straight portion 51, and the pins 45 are disposed at the ends of the curved portions 46 of the respective slots 34- remote from their straight portions 47. In the event of failure of the display of information provided by the cathode-ray tube it is required to use the collimator 13 to project the stand-by display of information provided by the unit lil on to the partially-transparent reflector. This is effected by moving the arm 34 towards the position shown in full outline in FIG. 1.

This movement of the arm 34 causes the sleeve 43 to rotate about the longitudinal axis of the tube 15 and thereby causes the pins 45 to move along the slots 44 and the pin 4% to move along the slot 49. During the first part of the movement of the arm 34 the pins 45 move in the curved portions 46 of the respective slots 44 and thereby cause the unit 10 to move away from the washer 27. At the same time the pin 48 moves along the curved portion 50 of the slot 49, but since the arcuate path through which the pin 48 is moved is also centered on the longitudinal axis of the tube 15 the unit 11 remains stationary. This action continues until the pins 45 reach the ends of the curved portions 46 of the slots 44 when it is arranged that the pin 48 reaches the end of the curved portion 50 of the slots 49.

During the remainder of the movement of the arm 34, the pins 45 move in the straight portions 47 of the slots 44 and, since the portions 47 he in a plane extending normally to the longitudinai axis of the tube 15, the pins 45 serve merely to maintain the unit 10 in the selected position. However, during this part of the movement of the arm 34-, the pin 48 moves in the straight portion 5'1 of the slot 49 and causes the unit 11 to be angularly displaced about the pin 23 and moved into the space between unit M1) and the washer 27. The pin 213 moves in one direction along the straight portion 51 to effect the first half of the movement of the unit 11, the second half of this movement being effected by the pin 48 moving in the opposite direction along the straight portion SI.

Accidental displacement of the arm 34 from the position shown in the drawings is prevented by a spring catch 52 mounted on the housing 26.

The device 29 is'shown in detail in FIGS. 5 to 7 and includes a base 75 of insulating material which is in the form of a disc and through which extends a plurality of circumferentially-spaced electrical conducting pins 76 to 83. The upper ends of the pins 76 to 83 are bent outwardly to provide a support for a disc 84 of ceramics which carries three filaments 85 to 87 of, for example, nichrome wire. The ends of the pins 76 and 80 extend through diammetrically opposed holes in the disc 84 and the portions of these pins projecting above the disc 84 are flattened to secure the disc 84 in position. The disc 84 has formed therein a plurality of holes which are disposed to form, on the upper face of the disc 84, an outline generally similar to that of the stand-by display of information shown in FIG. 3. The filaments 85 to 87 are threaded through respective groups of these holes so that the filaments 85, 86 and 87 are disposed to provide respectively a left-hand portion, a center portion, and a right-hand portion of the display of information. FIG. 7 shows the filament 86 threaded through holes in the disc 84. The ends of the filaments 85 to 87 are electrically connected to respective pairs of the pins 77 to 79, and 81 to 83 as indicated in FIG. 6. The disc 84 is housed within a glass envelope 88 which is sealingly connected to the circumferential edge of the base 75. The glass envelope 88 has a fiat top which is disposed near to the disc 84 and extends parallel therewith. The electrically-operated display device 29 is either evacuated or gas filled. When the filaments 85 to 87 are heated to incandescence by a low voltage electric current, the stand-by display of information provided by the filaments 85m 87 is in dashed outline and is visible through the fiat top of the envelope.

The electrically-operated display device 29 is mounted within the unit 11 with the flat top of the envelope 88 extending into an aperture in the plate 21 so that the external surface of the flat top and the external surface of the plate 21 lie in the same plane.

An alternative form of electrically-operated display device for providing the stand-by display of information is shown in FIGS. 8 and 9. Referring to FIGS. 8 and 9, the electrically-operated display device includes a rectangular panel 89 of ceramics having holes through which the filaments 85 to 87 are threaded to form an outline identical to that provided by the filaments 85 to 87 in the electrically-operated display device of FIGS. 5 and 6. The panel 89 is vertically mounted on a base 90 and is supported at its ends by supports 91. The panel 89 is housed within a glass envelope 92 of rectangular cross-section which is sealingly connected to the peripheral edge of the base 90. The panel 89 is disposed near to a side wall of the envelope 92 and extends parallel therewith. The base 90 carries a pair of electrical conducting pins 93 and 94 to which the ends of the filaments 85 to 87 are appropriately connected to enable the filaments 85 to 87 to be heated to incandescence by a low voltage electric current.

The filaments 85 to 87 may be constituted by a printed circuit formed on the disc 84 or the panel 89.

We claim:

' 1. An electrically-operated display device comprising a gas-tight housing having a transparent wall portion of substantially planar form, a substantially planar panel mounted within the housing to have a surface thereof in face to face opposition and close proximity to said planar wall portion, said panel having a plurality of pairs of holes spaced one from the other across the said surface of the panel, an electric filament carried by the said panel to extend across said surface of the panel so as to define intermediate the panel and the said planar wall portion a symbol to be displayed, said filament comprising an electric resistance element of elongate form threaded through the pairs of holes so that successive spaced parts of the element length extend across said surface, said parts of the element defining discrete portions of the said symbol, and electric terminal means carried by said housing for enabling the filament to be energized from outside the housing to provide an illuminated display of said symbol through the said planar wall portion.

2. A display device according to claim 1, wherein the gas-tight housing includes a base of substantially planar form and a plurality of electric terminals mounted on said base to extend through the base, the terminals being electrically connected to respective ends of the electric filament.

3. A display device according to claim 2, wherein the said panel extends substantially parallel to the base of the housing.

4. A display device according to claim 2, wherein the said panel is mounted on the electric terminals.

5. A display device according to claim 2, wherein the said panel extends transversely to the base of the hous- 6. A display device according to claim 5, wherein the said panel is directly mounted on the base of the hous- 7. A display device according to claim 1, including a plurality of electric filaments each of which is carried by the said panel.

8. A head-up display apparatus for a craft, comprising a display-producing means, a partially-transparent reflector, and collimator means for projecting on to the reflector the display provided by the display-producing means so that the display is visible to an observer in the craft viewing a distant scene through the reflector, wherein the display-producing means comprises a gastight housing having a transparent wall portion of substantially planar form, a substantially planar panel mounted within the housing to have a surface thereof in face to face opposition and close proximity to said planar wall portion, said panel having a plurality of pairs of holes spaced one from the other across the said surface of the panel, an electric filament carried by the said panel to extend across said surface of the panel so as to define intermediate the panel and the said planar wall portion a symbol to be displayed, said filament comprising an electric resistance element of elongate form threaded through the pairs of holes so that successive spaced parts of the element length extend across said surface, said parts of the element defining discrete portions of the said symbol, and electric terminal means carried by said housing for enabling the filament to be energized from outside the housing to provide an illuminated display of said symbol through the said planar wall portion.

9. A head-up display apparatus for a craft, including two display-producing means, a partially-transparent reflector, and collimator means for projecting on to the reflector the display provided by a selected one of the display-producing means so that the display projected is visible to an observer in the craft viewing a distant scene through the reflector, wherein the apparatus also includes means for effecting the selection of the particular display-producing means involved, said selection means including means for co-operating with the display-producing means to move the selected displayproducing means to the collimator means and to move the other display-producing means from the collimator means, and wherein one of the display-producing means comprises electrically-operated display device comprising gas-tight housing having a transparent wall portion of substantially planar form, a substantially planar panel mounted within the housing in face to face oPposition and close proximity to said planar wall portion, an electric filament carried by the said panel which at least along a part of its length extends across the panel intermediate the panel and the said planar wall portion in the form of a symbol to be displayed, and electric terminal means carried by said housing for enabling the filament to be energised from outside the housing to provide an illuminated display of said symbol through the said planar wall portion.

10. An apparatus according to claim 9, wherein said means for co-operating with the display-producing means comprises first means for moving said selected display-producing means and second means for moving said other display-producing means, the movement of the two display-producing means being effected sequentially.

M. An apparatus according to claim 9, wherein said means for co-operating with the display-producing means includes first means for moving a first of the display-producing means longitudinally from the collimator means so as thereby to provide a space between this display-producing means and the collimator means, and second means for moving a second of the displayproducing means into the said space along a path substantially normal to the direction of said movement of the first display-producing means.

112. An apparatus according to claim ill, wherein the said path is an arcuate path.

)13. An apparatus according to claim 9, wherein one of the display-producing means provides a main display of information and the other display-producing means provides an auxiliary display of information that can be used in the event of failure of the main display.

14. An apparatus according to claim llll, wherein said first display-producing means is arranged to provide a main display of information and said second displayproducing means is arranged to provide the auxiliary display of information that can he used in the event of failure of the main display.

i5. An apparatus according to claim lli, wherein the said second display-producing means comprises the said electrically-operated display devicev 

1. An electrically-operated display device comprising a gastight housing having a transparent wall portion of substantially planar form, a substantially planar panel mounted within the housing to have a surface thereof in face to face opposition and close proximity to said planar wall portion, said panel having a plurality of pairs of holes spaced one from the other across the said surface of the panel, an electric filament carried by the said panel to extend across said surface of the panel so as to define intermediate the panel and the said planar wall portion a symbol to be displayed, said filament comprising an electric resistance element of elongate form threaded through the pairs of holes so that successive spaced parts of the element length extend across said surface, said parts of the element defining discrete portions of the said symbol, and electric terminal means carried by said housing for enabling the filament to be energized from outside the housing to provide an illuminated display of said symbol through the said planar wall portion.
 2. A display device according to claim 1, wherein the gas-tight housing includes a base of substantially planar form and a plurality of electric terminals mounted on said base to extend through the base, the terminals being electrically connected to respective ends of the electric filament.
 3. A display device according to claim 2, wherein the said panel extends substantially parallel to the base of the housing.
 4. A display device according to claim 2, wherein the said panel is mounted on the electric terminals.
 5. A display device according to claim 2, wherein the said panel extends transversely to the base of the housing.
 6. A display device according to claim 5, wherein the said panel is directly mounted on the base of the housing.
 7. A display device according to claim 1, including a plurality of electric filaments each of which is carried by the said panel.
 8. A head-up display apparatus for a craft, comprising a display-producing means, a partially-transparent reflector, and collimator means for projecting on to the reflector the display provided by the display-producing means so that the display is visible to an observer in the craft viewing a distant scene through the reflector, wherein the display-producing means comprises a gas-tight housing having a transparent wall portion of substantially planar form, a substantially planar panel mounted within the housing to have a surface thereof in face to face opposition and close proximity to said planar wall portion, said panel having a plurality of pairs of holes spaced one from the other across the said surface of the panel, an electric filament carried by the said panel to extend across said surface of the panel so as to define intermediate the panel and the said planar wall portion a symbol to be displayed, said filament comprising an electric resistance element of elongate form threaded through the pairs of holes so that successive spaced parts of the element length extend across said surface, said parts of the element defining discrete portions of the said symbol, and electric terminal means carried by said housing for enabling the filament to be energized from outside the housing to provide an illuminated display of said symbol through the said planar wall portion.
 9. A head-up display apparatus for a craft, including two display-producing means, a partially-transparent reflector, and collimator means for projecting on to the reflector the display provided by a selected one of the display-producing means so that the display projected is visible to an observer in the craft viewing a distant scene through the reflector, wherein the apparatus also includes means for effecting the selection of the particular display-producing means involved, said selection means including means for co-operating with the display-producing means to move the selected display-producing means to the collimator means and to move the other display-producing means from the collimator means, and wherein one of the display-producing means comprises electrically-operated display device comprising gas-tight housing having a transparent wall portion of substantially planar form, a substantially planar panel mounted within the housing in face to face oPposition and close proximity to said planar wall portion, an electric filament carried by the said panel which at least along a part of its length extends across the panel intermediate the panel and the said planar wall portion in the form of a symbol to be displayed, and electric terminal means carried by said housing for enabling the filament to be energised from outside the housing to provide an illuminated display of said symbol through the said planar wall portion.
 10. An apparatus according to claim 9, wherein said means for co-operating with the display-producing means comprises first means for moving said selected display-producing means and second means for moving said other display-producing means, the movement of the two display-producing means being effected sequentially.
 11. An apparatus according to claim 9, wherein said means for co-operating with the display-producing means includes first means for moving a first of the display-producing means longitudinally from the collimator means so as thereby to provide a space between this display-producing means and the collimator means, and second means for moving a second of the display-producing means into the said space along a path substantially normal to the direction of said movement of the first display-producing means.
 12. An apparatus according to claim 11, wherein the said path is an arcuate path.
 13. An apparatus according to claim 9, wherein one of the display-producing means provides a main display of information and the other display-producing means provides an auxiliary display of information that can be used in the event of failure of the main display.
 14. An apparatus according to claim 11, wherein said first display-producing means is arranged to provide a main display of information and said second display-producing means is arranged to provide the auxiliary display of information that can be used in the event of failure of the main display.
 15. An apparatus according to claim 11, wherein the said second display-producing means comprises the said electrically-operated display device. 